Hello,
OK, thanks for your answer.
I give you some command to try to launch manually seahub.
# Run seahub in forground
sed -i "s/daemon = True/#daemon = True/g" /opt/yunohost/seafile/conf/gunicorn.conf
# Create environnement to start manually seahub
cd /opt/yunohost/seafile
sudo -u seafile -s
export CCNET_CONF_DIR=/opt/yunohost/seafile/ccnet
export SEAFILE_CONF_DIR=/home/yunohost.app/seafile-data
export SEAFILE_CENTRAL_CONF_DIR=/opt/yunohost/seafile/conf
export PYTHONPATH=/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib/python2.6/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib64/python2.6/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seahub:/opt/yunohost/seafile/seafile-server-7.0.5/seahub/thirdpart:
export PYTHONPATH=/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib/python2.7/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib64/python2.7/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib/python2.6/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seafile/lib64/python2.6/site-packages:/opt/yunohost/seafile/seafile-server-7.0.5/seahub:/opt/yunohost/seafile/seafile-server-7.0.5/seahub/thirdpart:
# Try to launch seahub
python2.7 /opt/yunohost/seafile/seafile-server-7.0.5/seahub/thirdpart/gunicorn seahub.wsgi:application -c /opt/yunohost/seafile/conf/gunicorn.conf --preload --log-level debug --error-logfile /tmp/seahub-error.log
# Get the content of log
cat /tmp/seahub*.log